beams and magnetized plasmas jean-pierre boeuf ...bpw anu 30/05-03/06/2013. beams and magnetized...

1

GREPHE BPW ANU 30/05-03/06/2013

Beams and magnetized plasmas

Jean-Pierre BOEUF

LAboratoire PLAsma et Conversion d’Energie

LAPLACE/GREPHE

CNRS, Université Paul SABATIER, TOULOUSE

2

GREPHE BPW ANU 30/05-03/06/2013


Ion acceleration and electron transport through a magnetic barrier Principle of positive ion acceleration through a magnetic barrier

Collisional & turbulent EXB electron transport in a magnetic barrier

Illustration of plasma turbulence with simple 1D PIC

Negative ion sources for neutral beam injection Principles of NIS for NBI

Plasma transport across the magnetic filter in a negative ion source

Plasma rotation in an e-beam sustained magnetized plasma column

Conclusion

Outline

3

GREPHE BPW ANU 30/05-03/06/2013








Conclusion

Outline

4

GREPHE BPW ANU 30/05-03/06/2013

Beams and magnetized plasmas Principle of positive ion acceleration through a magnetic barrier

Ion acceleration through a magnetic field barrier

Magnetic barrier = B field ^ to electron path from cathode to anode

~ 300 V between emissive cathode (no cathode sheath) and anode

Drop of electron conductivity in magnetic field barrier

→ Large electric field Ion extraction and acceleration

B

E

an

od

e

cathode (electron emission)

plasma

ions

electrons

EXB drift must be closed (azimuthal symmetry)

Hall thrusters, ion sources for processing

5

GREPHE BPW ANU 30/05-03/06/2013


0.00 0.02 0.04 0.06 0

1

2

3

Ele

ctr

ic F

ield

(10

4 V

/m)

Position (cm)

0

50

100

150

200

B r

E x

exhaust plane

Ma

gnetic F

ield

(G

auss)

0.00 0.02 0.04 0.06 0

100

200

300

Pote

ntial (V

)

Position (cm)

0

5

10

15

acceleration

ionization

S

V

exhaust plane

Ioniz

ation (

10

23 m

-3 s

-1)

Electron drift in the azimutal direction:

Hall current // EXB

Magnetic barrier is efficient because of

closed drift in azimutal direction

x

E

B EXB

Hall Thruster

6

GREPHE BPW ANU 30/05-03/06/2013


PPS 20k ML, SNECMA – CNRS – CNES, Euopean project HiPER

20 kW Hall Thruster

7

GREPHE EPS 2008 Hersonissos


Br

Ex

x

y=Rq

Amplitude of the azimutal field

~ 0.2-0.4 axial field

Wavelength ~ larmor radius

Eq (V/cm)

-200

-100

0

100

200

0 1 2 3

5 4 3 2 1 0

Azim

uta

l P

ositio

n R

q (

mm

)

Axial Position X (cm)

2D PIC simulations predict

azimuthal instability

J.C. Adam et al., Physics of Plasmas 11, 295 (2004)

8



Large azimutal drift velocity in the exhaust region instability plasma turbulence

Short wavelength close to electron gyroradius

Velocity spread comparable to EXB drift velocity

Generated by turbulence

Dispersion equation of electrostatic waves in a hot

magnetized electron beam • Cold, non magnetized ions

• Kinetic description of magnetized electrons

• Drift velocity not much smaller than thermal velocity

0 1 2 3

Axial Position (cm)

Vx

Vz

Quasi linear theory gives resonances at dkV n

dkV n

A Ducroq et al. Physics of Plasmas, 13, 102111 (2006)

x107 m/s

1.

0.

–1.

1.

0.

–1.

Azimuthal drift instability - theory

9


Beams and magnetized plasmas Collisional & turbulent EXB electron transport in a magnetic barrier

Theory + simulation predict that transport across B is enhanced by turbulent azimuthal E field

Realistic (and simpler) models of Hall Thrusters need an estimation of electron mobility

Can we define an electron mobility in the conditions of a Hall thruster ?

10

GREPHE BPW ANU 30/05-03/06/2013


1D PIC MCC model (azimuthal, EXB direction)

- Given E, B, plasma density, gas density

- Particle-In-Cell Monte Carlo Collisions

- 3D-3V trajectories but Poisson’s equation in ExB direction only

- Collisions included, ionization treated as excitation

B

Ex

x (axial)

periodic boundary conditions EXB direction

y (azimuthal)

z (radial)

11

GREPHE BPW ANU 30/05-03/06/2013


Ey

ne ni


turbulence in azimuthal direction

B

Ex EXB

L=0.5 cm

Ex=100 V/cm – B=100 Gauss – n=1016 m-3 – p=0.01 torr

y

70 V/cm

12

GREPHE BPW ANU 30/05-03/06/2013



turbulence in azimuthal direction

E

ni ne

E

ni ne

B

Ex EXB

Ex =100 V/cm – B=100 Gauss – n=5x1016 m-3 – p=0.02 torr

L=1 cm L=2 cm

y

y y

13

GREPHE BPW ANU 30/05-03/06/2013



0.01 0.1 1

1

10

E=104 V/m; B=10 mT

Mobili

ty (

m2/V

/s)

Pressure (torr)

classical

PIC, n=1017

m-3

PIC, n=1016

m-3

- Turbulence appears around 0.1 torr (/n>~2)

- Turbulent mobility depends on plasma density

- No solutions below ~0.01 torr (depends on n)

- Real operating conditions much below 0.01 torr

(gas density 1012 – 1013 m-3 )

Question: can we define a mobility in the

conditions of a Hall thruster if we include

wall losses (momentum and energy)

thruster

Classical mobility

Electron mobility can be deduced from PIC model and compared with classical mobility

2 2e

e

m

n

n

14

GREPHE BPW ANU 30/05-03/06/2013








Conclusion

Outline

15

GREPHE BPW ANU 30/05-03/06/2013

Negative Ion Source for Neutral Beam Injection Principles of NIS for NBI

Magnetic barrier (or filter) is also used in hydrogen negative ion sources

Context of fusion applications

Heating of ITER plasma by high energy deuterium neutral beam (1 MeV)

Negative ions produced in a low temperature ICP plasma source

Ions are accelerated to 1 MeV, then neutralized and injected in ITER plasma

At such high energy negative ions easier to neutralize than positive ions

Magnetic filter used to limit electron energy and electron current extraction

16

GREPHE BPW ANU 30/05-03/06/2013


EC Electron Cyclotron 20 MW/CW, 170 GHz, 24 gyrotrons

IC Ion Cyclotron 20 MW/CW, 35-65 MHz

H-NB Heating-Neutral Beam 2 x 16.5 MW, 1 MeV, Deuterium

200 A/m2 , 3600 s

EC, IC, and H-NB heating systems, i.e. 73 MW, all required for the 1st phase of ITER

The Neutral Beam Injection system is essential for the ITER program

http://www.jet.efda.org/pages/focus/heating/index.html

17

GREPHE BPW ANU 30/05-03/06/2013


Negative Ion Source for the ITER NBI System

RF Inductively Coupled Plasma at 1 MHz

Must provide negative ions H-/D-, 40 A, 200 A/m2

Must operate at low pressure ~ 0.3 Pa

Co-extracted electron current < negative ion current

Current uniformity better than ±5%

The negative ion source is developped at IPP Garching

Complete Neutral Beam Injection system built in Padova

Source modeling (+ validation experiments) at LAPLACE in Toulouse

18

GREPHE BPW ANU 30/05-03/06/2013


Requirements

1 MeV negative ions, 40 A, 200 A/m2 , current uniformity better than ±5% , pulse duration 3600 s

pressure ~ 0.3 Pa, ICP 100 kW, 1 MHz

co-extracted electron current < extracted negative ion current

filter field

N

S

Driver Expansion

Region

Extraction

Region

N

S

S

E. S

peth

et al, N

ucl. F

usio

n 4

6 S

220 (

2006)

H- H2

grids

19

GREPHE BPW ANU 30/05-03/06/2013

Source geometry and Magnetic Filter

Negative Ion Source for Neutral Beam Injection Plasma transport across the magnetic filter

driver filter expansion

bias

extraction

Given absorbed power in driver

Collisions with neutrals included

(elastic, excitation, ionization)

e-i Coulomb collisions included

Simulations performed at lower densities

(scaling assumed, Debye sheath not resolved)

JP Boeuf, J Claustre, B Chaudhury, G Fubiani, Phys Plasmas 19 ,113510 (2012)

G Fubiani, G J M Hagelaar, St Kolev and J-P Boeuf , Phys. Plasmas 19, 043506 (2012)

B

2D PIC MCC model of negative ion source

20

GREPHE BPW ANU 30/05-03/06/2013


Plasma density and plasma potential

Plasma not uniform along extracting grid due to diamagnetic currents

Electron Density

1018 m-3

P=80 kW/m

5 1017

2 1017

42 V

36 V

31 V

Plasma Potential

20 V bias

Biased

plasma grid


21

GREPHE BPW ANU 30/05-03/06/2013


Electron Current Density Distribution no negative ions

Electron Current Density from PIC MCC simulations

Chamber walls perpendicular to JXB

Magnetic barrier not as efficient as in

closed drift geometry (e.g. Hall thrusters)

Large electron current through filter

Scales as 1/B

Transport across B is strongly affected

(and controlled) by the presence of walls


22

GREPHE BPW ANU 30/05-03/06/2013


Positive Ion Current Density Distribution no negative ions

Positive Ion Current Density from PIC MCC simulations

Ions are only weakly magnetized

v

v


23

GREPHE BPW ANU 30/05-03/06/2013


Understanding Electron Current Density Distribution

Large electron pressure gradient at the

entrance of the filter

e en kT B large in the filter

Diamagnetic electron current large in the filter

Because of walls perpendicular to diamag current,

generation of E field // and opposing diamagnetic current

→ asymmetry of plasma

→ EXB current through filter

Electron Pressure: Pe=nekTe

e en kT

e en kT B

B


24

GREPHE BPW ANU 30/05-03/06/2013


Electron Current Density Distribution

Electron Current Density from PIC MCC simulations

Chamber walls perpendicular to JXB

Magnetic barrier not as efficient as in

closed drift geometry (e.g. Hall thrusters)

Large electron current through filter

Scales as 1/B

Transport across B is strongly affected

(and controlled) by the presence of walls


25

GREPHE BPW ANU 30/05-03/06/2013

Negative Ion Source for Neutral Beam Injection Plasma rotation in an e-beam sustained magnetized plasma column

New source under investigation

New Neutral beam Injection system (for DEMO) based on photo-neutralization of negative ions

Proposed by CEA Cadarache (A. Simonin)

Requires a long and thin source to produce an intense beam sheet

Magnetized plasma column (uniform B field)

Plasma generated by filaments in a first phase, ICP or helicons in a second phase

Better uniformity ? Plasma rotation ?

Simonin et al.

Nucl. Fusion 52 (2012) 063003

26

GREPHE BPW ANU 30/05-03/06/2013



New Neutral beam Injection system (for DEMO) based on photo-neutralization of negative ions

Proposed by CEA Cadarache (A. Simonin)

Requires a long and thin source to produce an intense beam sheet

1 m

filaments

grids B

ICP or

helicons

grids B

Simonin et al., Nucl. Fusion 52 (2012) 063003

20 cm

27

GREPHE BPW ANU 30/05-03/06/2013


Simonin et al., Nucl. Fusion 52 (2012) 063003

28

GREPHE BPW ANU 30/05-03/06/2013



Similarities with magnetized plasma columns studied in different labs

e.g. magnetized plasma column MISTRAL at the PIIM lab in Marseille, france

limiter bias

29

GREPHE BPW ANU 30/05-03/06/2013



Similarities with magnetized plasma columns studied in different labs

e.g. magnetized plasma column MIRABELLE at IJL, Nancy, France

30

GREPHE BPW ANU 30/05-03/06/2013


S. Jaeger, N. Claire, C. Rebont, Phys. Plasmas 16, 022304 (2009)

Observation of EXB rotating instability (~5 KHz), m=1 or m=2 mode

Argon, 0.02 Pa, B=16 mT, 50 eV e-beam, 1 m column length, limiter 8 cm diameter

31

GREPHE BPW ANU 30/05-03/06/2013


Magnetized plasma column studied in Marseille – PIIM Lab

C. Rebont, N. Claire, Th. Pierre, and F. Doveil, PRL 106, 225006 (2011)

Measured plasma density (probes) Measured Ion velocity (LIF)

m=2 mode, LIF measurements of ion velocity and electric field

32

GREPHE BPW ANU 30/05-03/06/2013


B

2D simulation domain, B^ to simulation domain

3D, 3V trajectories

2D Poisson (assumption of uniform column – flute mode)

Charged particle losses in the B direction included o Bohm losses for ions frequency: 2UB/L

o Electron losses when electron reaches end plates and if

energy in the B direction larger than potential difference

between plasma and end wall

o Grid: negative bias – Limiter and walls grounded

2D PIC MCC model of magnetized plasma column

X

33

GREPHE BPW ANU 30/05-03/06/2013


f

Time averaged potential distribution


0

1

34

GREPHE BPW ANU 30/05-03/06/2013


Time averaged plasma density distribution


0

1

35

GREPHE BPW ANU 30/05-03/06/2013


Time averaged electron temperature distribution


0

1

36

GREPHE BPW ANU 30/05-03/06/2013


f

No steady state solution

Rotating Instability – Rotation in about 200 s

o Plasma density o Electric Potential


0

1

n (1014 m-3 )

37

GREPHE BPW ANU 30/05-03/06/2013


No steady state solution

Rotating Instability

o Distribution of ion velocity o Electric Potential o Plasma density

Ion velocity tangent to limiter edge in plasma arm (as in experiments)

Ion velocity perpendicular to limiter edge ahead of plasma arm (as in experiments)

Ion velocity follows EXB

Rotating Instability (Modified Simon-Hoh ?) + Kelvin Helmhotlz structures


38

GREPHE BPW ANU 30/05-03/06/2013


t = s


0

1 f (2.5 V) n (1014 m-3 )

o Electric Potential o Plasma density

39

GREPHE BPW ANU 30/05-03/06/2013




t = s

f (2.5 V) n (1014 m-3 )

0

1

40

GREPHE BPW ANU 30/05-03/06/2013




t = s

f (2.5 V) n (log, 1014 m-3 )

0

1

41

GREPHE BPW ANU 30/05-03/06/2013


Ion acceleration and electron transport through a magnetic barrier Very simple and appealing concept

Very complex and non-linear operation

Turbulence and plasma-wall interaction both important

Can we define an electron mobility ?

Negative ion sources for neutral beam injection Magnetic filter with non-closed EXB or XB path induces assymetry and leaks

2D PIC model improve understanding of rotating magnetized plasma column

Conclusions

42

GREPHE BPW ANU 30/05-03/06/2013


0

1

n (1014 m-3)

n (log, 1014 m-3)

f (2.5 V)

Te (5 eV)

43

GREPHE BPW ANU 30/05-03/06/2013


ne (log, 1014 m-3) ni (log, 1014 m-3)

0

1

Electron and ion densities (log)

beams and magnetized plasmas jean-pierre boeuf ...bpw anu 30/05-03/06/2013. beams and magnetized...

Documents